1. FIELD OF THE INVENTION
This invention relates to variable semiconductor capacitance transducers and in particular to capacitance transducers adapted to measure pressure variations which include an integrated semiconductor reference capacitance transducer.
2. DESCRIPTION OF PRIOR ART
Various semiconductor variable capacitance pressure transducers have been constructed. Basically, they disclose forming one plate of a parallel plate capacitor of a semiconductor material with the other plate being a dielectric such as quartz. Generally, oppositely disposed areas on the respective plates are metallized to provide the conductive regions of the capacitor. Doping the semiconductor plate with a high impurity concentration has also been utilized to form a conductive region in the semiconductor capacitor plate. Typically, one semiconductor plate includes a thin diaphragm portion which deflects in response to a pressure differential across it.
The utility of such semiconductor pressure transducers in the microbar pressure range is greatly limited by changes in capacitance due to thermal effects. Temperature changes cause variation in plate separation due to thermal expansion or contraction of the materials between the plates. Plate area is also varied by the thermal expansion or contraction of the plates. These variations in plate separation and area are called thermal offset. Temperature changes also cause deflection of the diaphragm due to stress parallel to the surface of the plates. The stress results from differences in the coefficients of thermal expansion of the dielectric plate and the semiconductor plate. Such stress is termed thermal stress. Thermal stress increases with the thickness of the dielectric above the plates and with the area of the surface of the dielectric plate. Correction of these thermal effects problems has been left to extensive calibration and the selection of dielectrics with coefficients of thermal expansion similar to the semiconductor material.
Such calibration is expensive and time consuming and even if the coefficient of thermal expansion of the dielectric is similar to the semiconductor material, thermal off-set is still a problem. Additionally, when one plate of the capacitor is a dielectric, metallization of the dielectric surface requires additional processing in order to provide a conductive area thereupon.
Thus, the sensitivity and utility of capacitive pressure transducers would be greatly increased if the thermal effects on a capacitive transducer could be distinguished from the effects of pressure thereon. Batch processing and the fabrication of integrated circuits which included capacitive transducers would be greatly facilitated if both plates of a capacitive transducer could be made of the same semiconductor material, thus requiring only one processing line. Thermal stress would be reduced if thick dielectric materials could be eliminated from the transducer.